1. Field of the Invention
The present invention generally relates to an optical apparatus and in particular but not exclusively an optical apparatus having a function of improving image shake.
2. Description of the Related Art
Many optical apparatuses, (e.g., cameras and interchangeable lenses), have vibration isolation systems for reducing image shake caused by hand motion or the like.
Hand motion is normally a vibration with a frequency of about 1 to 10 Hz. In an optical apparatus having a typical vibration isolation system for taking a picture without blurring even when hand motion occurs during exposure, vibration of the optical apparatus is detected. Then, a correcting lens is moved along a plane perpendicular to an optical axis in accordance with the detection result (optical vibration isolation), or an area to be extracted (output) from a shot image is shifted in accordance with the detection result (electronic vibration isolation).
The vibration caused by hand motion or the like is determined by detecting an acceleration, an angular acceleration, an angular velocity, an angular displacement, etc., with a laser gyroscope or the like and performing calculations based on the detection results. Then, the image shake is improved by driving the correcting lens or shifting the output area in the shot image on the basis of shake information obtained by the calculations.
In general, when a shooting magnification is 0.1 or less, the image shake can be sufficiently corrected simply by correcting angular shake in a direction an imaging plane tilts. However, when the shooting magnification is more than 0.1, the influence of parallel shake along the imaging plane is relatively large. In addition, as the shooting magnification increases, the influence of shake in a focal direction (direction of a shooting optical axis) also increases. This is because the parallel shake causes an image shift proportional to the shooting magnification on the image plane and the shake in the focal direction (hereafter called focal shake) causes a focus shift proportional to the square of the shooting magnification.
Accordingly, in optical apparatuses, (e.g., ones having macro lenses and having high shooting magnifications), the parallel shake may be corrected by controlling an optical shake correction unit using an acceleration sensor.
On the other hand, an acceleration detected by an acceleration sensor includes both the acceleration caused by the parallel shake and the acceleration of gravity. In addition, in order to calculate the velocity and displacement from the detected acceleration, it is necessary to determine an accurate initial velocity. For this purpose, Japanese Patent Laid-Open No. 9-218435 discloses a method described below.
First, a direction of gravitational acceleration is measured for a predetermined time interval, and an average direction of gravitational acceleration is determined from the average of the detection result. Then, a displacement in an angular direction is determined from the output of an angular velocity sensor, and a displacement in the direction of gravitational acceleration is calculated. Then, a shake acceleration is calculated by subtracting the gravitational acceleration from the detected acceleration.
The above-mentioned Japanese Patent Laid-Open No. 9-218435 also discloses a method for calculating a velocity using an acceleration between a first peak and a last peak within a predetermined time interval of the acceleration detected by the acceleration sensor by assuming that a shake displacement is the same between the first and last peaks.
In addition, methods for directly determining the shake displacement from the displacement of the image are also suggested. For example, camera shake can be detected from the shake of the image on an AF sensor.
In conventional methods, in which the shake displacement is calculated from the acceleration, it is necessary to determine the direction of gravity by measuring the acceleration for a long time. Therefore, it requires a long time to obtain the shake displacement, which reduces the snap-shooting ability of shooting apparatuses.
In addition, in this method, the initial velocity is calculated on the basis of the assumption that the camera shake is a periodic motion. However, the actual camera shake is not periodic, and therefore the calculated initial velocity includes a large error.
As described above, the camera shake displacement may also be determined from the output of a photoelectric transducer, such as an AF sensor, and the image shake may be corrected on the basis of only the thus detected camera shake displacement. However, when this method is applied to a single lens reflex camera, although high correction accuracy can be obtained, the camera shake cannot be detected during exposure. This is because a quick return mirror is removed from a shooting optical path and an object image is not formed in the AF sensor during exposure.